Traditionally, radio telecommunication systems have been designed almost exclusively for voice or for packet data. There have been several attempts to design systems to provide both data and voice in the same system. One such proposal is the ETSI General Packet Radio Service (GPRS) which is designed for packet data transfer and is an overlay network on the circuit switched GSM system which is designed for speech communication. A GPRS architecture proposed by ETSI in Technical Specification 3.6 is shown in FIG. 1. Shown mainly on the left of the diagram is a conventional GSM mobile telephone system for full duplex voice communications comprising a Mobile Switching Centre (MSC) a Base Station System (BSS) usually including a Base Station Controller (BSC) and a Base Transceiver Station (FITS), and a mobile terminal (MT) and a Home Location Register (HLR). Packet data services are limited to the Short Message Service (SMS) which is dealt with by an SMS Gateway Mobile Switching Centre (SMS-GMSC) and a Short Message Service Centre (SM-SC). Fax is dealt with as in an ordinary telephone system, e.g. via suitable modems and an Interworking Function (IWF) fax data is transmitted via circuit switching. Hence, conventional mobile telecommunications systems generally use what may be described as circuit switched data transmissions. GPRS adds two new nodes to such a system, namely the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support node (GGSN), both of which may be seen as routers. The SGSN contains the identity of MT in its routing tables which are inserted when the MT registers with the network. The GGSN is connected to other data carrying networks, for example a Packet Data network (PDN), for the receipt and transmission of packets of data. As the GPRS system is in parallel to the GSM system information about change of location of the MT is also sent to the SGSN/GGSN.
The above hybrid system may be adapted to a Third Generation Mobile Telephone system such as the UNITS system as shown schematically in FIG. 2. Further details of such an implementation may be found in the book by Ojanperá and Prasad, “Wideband CDMA for Third Generation Mobile Communications”, Artech House Publishers, 1998. Basically, the Radio Access Network (RAN) provides the network-side equipment for communicating with the MT. A GPRS SGSN and a UMTS MSC are provided in parallel between the RAN and the relevant network, i.e. or a PDN or a Public Service Telephone Network (PSTN), respectively.
For multimedia and especially highly interactive wireless applications there can be a wide variation in the amount of data to be sent in one direction as well as in the rate at which replies to the data are expected. Further, there is a general interest in providing services at difference priorities and at different prices. Thus, the GPRS standards provide, especially in ETSI standard 3GPP TS 08.18 (e.g. V8.10.0 (2002-05)), possibilities to dynamically adjust the Quality of Service (QoS) for data transmitted over the air interface.
GPRS provides a connectionless support for data transmission. However, in order to use the scarce resources on the radio air interface between the BTS and the MT, a circuit switched radio resource allocation is used. Thus, although the networks attached to the GGSN may operate in a completely connectionless way, the transmission of the data packets across the air interface makes use of conventional timeslot and frame management. Accordingly, at some position in the GPRS network a packet handler is required which prepares the packets for transmission in frames across the air interface and receives the frames from the air interface and prepares them for transmission to the data network. This unit may be called a Packet Control Unit (PCU) and may be placed at several alternative positions, e.g. in the Base Transceiver Station (BTS), in the Base Station Controller (BSC) or between the BSC and the SGSN. Generally, the PCU may be assigned to some part of the BSS—the base station system. Typically frame relay will be used between the PCU and the SGSN.
Referring to FIGS. 1 and 2, in order to access GPRS services, a user equipment (UE) such as a mobile terminal (MT) or mobile phone first performs a GPRS attachment. This operation establishes a logical link between the UE and the SGSN, and makes the UE, available for SMS (Short Message Services) over GPRS, paging via SGSN, and notification of incoming GPRS data. Also the authentication of the user is carried out by the SGSN in the GPRS attachment procedure. In order to send and receive GPRS data, the UE activates the packet data address wanted to be used, by requesting a PDP activation procedure (Packet Data Protocol). This operation makes the UE known in the corresponding GGSN, and interworking with external data networks can commence. More particularly, a PDP context is created in the UE, the GGSN and the SGSN. The packet data protocol context defines different data transmission parameters, such as the PDP type (e.g. X.25 or IP), the PDP address (e.g., X.121 address), the quality of service (QoS) and the NSAPI (Network Service Access Point Identifier). The UE activates the PDP context with a specific message comprising the TLLI (Temporary logic link Identity), an Activate PDP Context Request, in which it gives information on the PDP type, the PPP address, the required QoS and the NSAPI, and optionally the access point name (APN). The SGSN provides the TLLI which identifies the UE.
The setting up of circuit switched calls across the air interface in a GPRS network is shown in message flows in FIGS. 3 and 4. In FIG. 3 a data request is initiated by a mobile terminal (MT) using an access control channel, e.g. a Random Access Channel RACH. When a MT has some data to send it makes an Uplink Radio. Connection Establishment Request specifying how much data is to be sent. The RAN replies with a confirmation message that the uplink radio link is provided and gives details of when and how the MT is to transmit, e.g. which timeslot and how much of the timeslot can be used. Then the data is transmitted by the MT on a traffic channel and the RAN disconnects the radio link after all data has been transmitted successfully. The data received by the RAN is forwarded to the SGSN and from there to the GGSN which removes any headers used for transporting the data up to this point and transfers the data to the relevant PDN, e.g. via the Internet to a remote server. As some time later the answer to the data arrives from the remote site, e.g. a service provider's server on the Internet. On receipt of this answer a downlink radio connection is set up by the RAN via a control channel and the answer data transferred via a traffic channel. After transfer the radio connection is released once again.
FIG. 4 shows a similar message scheme when the initiating message is downlink. Again, the downlink and uplink transfers are not coupled so that the downlink radio connection is released at the end of the downlink transmission and before the answering uplink transmission.
Data transmission over an air interface is subject to errors. For some packet data transmissions some guarantee of the received data is required. Traditionally this has been achieved by an automatic repeat request (ARQ) protocol in which ACK (accepted) and NACK (not accepted) messages returned depending on whether a received block of data was correctly received or not. Either the failure to receive an ACK message within a predetermined time or the receipt of a NACK message triggers resending of the data. A known problem with, such as scheme is setting an optimum time interval for receipt of an ACK message before the data is resent. Too short a time can result in data being resent frequently when an ACK message is still going to be received and would have stopped the resend. Too long a time can result in use of large buffers to accommodate data until the status of this data is clarified. U.S. Pat. No. 6,289,224 proposes a scheme in which the length of time for an outbound communication is determined and this is transmitted to the transmitting device so that the timer can be started. This known technique relies on the fact that the type of data to be sent is sensibly constant. However, in the type of applications mentioned above the data rates and answer frequencies can be very varied and the known scheme is not optimal in all circumstances.
It is an object of the present invention to provide a data carrying cellular mobile radio telecommunications system and a method of operating the same which provides an improved QoS.